At least this postage stamp-sized "flash" memory -- used to store all your camera's photos and videos -- has dropped in price considerably over the past few years.
But if you need some help in understanding the differences between all the options out there, here's a look at a few considerations.
Flash memory cards come in all shapes and sizes. The first thing you need to know is which one your digital camera takes, since all of them are incompatible with one another. The camera will say what kind of memory card is required on the box, in the manual and sometimes on the camera itself. The most popular format is called SecureDigital, or SD (and SDHC or SDXC, which hold a lot more photos), but there's also Memory Stick (usually for older Sony cameras), xD (some FujiFilm and Olympus cameras, though they've since switched to SD) and Compact Flash (a thicker card used in some professional cameras). Smartphones that accept removable cards usually take microSD, which is the size of your fingernail.
Capacity refers to how many photos (and videos) you can store on the card. The higher the capacity — such as 16 gigabytes (GB) over 4GB — the more files you can store on the card. For example, a 4GB card can store about 677 12-megapixel photos, therefore on a 16GB card you can fit about 2708 12-megapixel photos. If you want to fit more on the memory card, you can choose to shoot in lower-quality, but it's not recommended (now that memory is so cheap, there's no excuse). A higher capacity card is also idea for storing video clips, since most cameras allow you to shoot HD movies. Prices have dropped considerably for memory cards, so it's not unlikely to get a 16GB SDHC card for about $10.
Flash memory cards can vary in speed. The speed refers to how fast information is written to the card (while you're shooting pictures and videos) or when data is read from it (when it's inserted into a computer's card reader, for example). Therefore, the faster the memory card speed, the better: for example, 80x is better than 40x, and 150x is better than 80x. You might see it written as "Pro" series cards or measured by Class 2, 4 or 6, capable of a data transfer rate of 2, 4 or 6 megabits per second, respectively. Generally, the higher-speed cards will cost more. Pro photographers like faster cards as they might want to hold down the shutter button to take multiple shots at once, plus they're also ideal for shooting video because they will capture the maximum frames-per-second for smoother motion.
Some industry experts believe there is little difference in quality between aggressively priced memory cards by lesser-known brands and those produced by the more renowned manufacturers. Regardless of your preference, it's always a good idea to take note of the company's warranty before you purchase a memory card. The good news is even some of the lesser-known companies offer a lifetime warranty on their memory cards, but be sure to read the conditions ahead of time. If you are saving files on a memory card long-term, just to be on the safe side be sure to make a back-up of any data (onto, say, a hard drive). Also research the company's technical support setup -- look for a toll-free telephone number and reasonable hours of operation. Sometimes the retailer will offer an additional warranty to replace the memory card if it ceases to work.
Finally, flash memory cards may vary in extras. A SecureDigital (SD) card, for example, get its name because you can flick a small switch on its side or back to "secure" its contents (that is, protecting it from being accidentally erased). Some companies offer password-protection on a memory card — in case it's lost or stolen. Some cards, like the Eye-Fi models, can wirelessly beam photos and videos from the digital camera to a nearby computer (or directly upload them to various websites). In other cases, memory cards might come preloaded with optional computer software (such as anti-virus protection) or might come bundled with songs. Or the card might come with various adaptors, such as a USB dongle to easily connect it to a computer. You get the idea. While perusing the isles at your favorite electronics retailer, check the box for added value.